Ultracapacitors are electrochemical cells which act as high energy density capacitors that are often used in energy storage devices. Due to their higher capacitance per size and mass, ultracapacitors occupy significantly less space than conventional electrolytic capacitors, and thus, are often preferred for compact and critical installations. In a typical application, several identical ultracapacitor cells are connected in series to provide a potential difference which sufficiently matches that required by the load. Ideally, each ultracapacitor cell of the series would behave identically with the adjacent cells. In reality, however, due to manufacturing inconsistencies, each ultracapacitor cell tends to behave differently over time, resulting in undesirable performance issues. For instance, each ultracapacitor cell can vary in capacitance and leakage current, leading to an uneven voltage distribution across the cells and inconsistent rates of discharge.
Such inconsistencies are typically countered by incorporating a balancing scheme or circuit configured to equalize the voltage of each cell within the series and improve the overall storage capacity. One of such schemes is an active balancing scheme. Active balancing schemes employ comparators or other active elements which aim to substantially equalize the voltage across the series-connected ultracapacitor cells. The nonlinear behavior of active balancing schemes reacts well to effectively balance and evenly distribute the voltage among series-connected ultracapacitor cells. However, there are some notable drawbacks to active balancing schemes. In some situations, for instance, active balancing schemes can be overly complex for the application at hand, and in all cases, active balancing schemes come at an overall higher cost of implementation.
Typical series-connected ultracapacitor cells alternatively use passive balancing schemes to equalize any voltage distributions. As compared with active balancing schemes, passive balancing schemes are much simpler in concept and in design, and employ passive circuit elements, such as resistors, to balance or distribute any uneven voltage distribution among the ultracapacitor cells. A typical passive balancing scheme for a set of series-connected ultracapacitor cells refers to a single, fixed balancing voltage or a fixed voltage variation in controlling the voltage distribution. Passive balancing schemes operate by bleeding off or discharging some of the electrical energy from higher voltage cells in order to balance the voltage of those cells with those of the lower voltage cells. However, in doing so, passive balancing schemes undesirably waste energy during the balancing process. Furthermore, current passive balancing schemes fail to take individual cell-to-cell capacitance variations into account.
Accordingly, there is a need for a more effective as well as a more simplified approach to balancing series-connected ultracapacitor cells. More specifically, there is a need for a balancing scheme that is not only less costly to implement, but also reduces the amount of electrical energy that is lost during the balancing process. Furthermore, there is a need for a balancing scheme which considers variations in capacitance or voltage in each individual ultracapacitor cell.